Granted, the show was about a POW camp and not a concentration camp; regardless, it was mocking an extremely dark moment in the relatively recent past.
That's the crucial difference. Buchenwald was not a concentration camp like the other death camps. As a matter of fact the inmates rebelled and took over the camp from the Nazis prior to the arrival of the American troops!
For all I know Robert Clary might have been among those who initiated the uprise and therefore his participation in "Hogans Heroes" meant autobiographically more to him than just a TV show mocking Nazis...
If I remember correctly Gene Roddenberry served on a US bomber over Europe and therefore never really saw what was actually going on "down there".
In the case of Nimoy his tasteless line in "Patterns of Force" might be attributed to ignorance, but there is one who doesn't have a good excuse - William Shatner!
In 1961 he had a supporting role in "Judgment at Nuremberg
" which featured original, very graphic films of the death camps (a fantastic film with the finest actors Hollywood had to offer!). So unless he never saw the final film, I would have expected him - at least and of all the people - to draw a line and tell the diector of "Patterns of Force" he wouldn't continue shooting until Leonard's line had been changed.
Well, apparently that didn't happen.
"Get a life?"
Look who's talking! What's it good for once you have an opportunity to make a stand against ignorance and stupidity but instead decide to remain silent and let ignorance and stupidity prevail?